deepsubduction

Deep subduction is the process by which oceanic lithosphere continues to descend through the mantle beyond the shallow portions of subduction, potentially reaching the mantle transition zone (about 410 to 660 kilometers deep) and, in some cases, deeper into the lower mantle. It contrasts with shallower subduction, which typically involves the upper mantle and shallower depths. Deep subduction is studied through geology, geophysics, and geochemistry to understand how slabs behave at extreme pressures and temperatures.

Mechanisms and characteristics include slab-driven descent aided by slab pull and mantle convection, as well as the thermal and chemical evolution of the subducting crust. As slabs descend, they undergo high-pressure metamorphism, with minerals such as eclogite forming under great pressures. Dehydration of hydrous minerals within the slab releases fluids that alter the surrounding mantle, influencing melting, viscosity, and mantle convection. The geometry of descent can involve bending, tearing, or sustained penetration; some slabs appear to stagnate near the 660-kilometer boundary, while others appear to penetrate into the lower mantle in certain regions.

Evidence and interpretation come from seismic tomography, earthquake distributions, and studies of high-pressure metamorphic rocks. Seismic images reveal slabs dipping into the transition zone and, in some places, showing continuation into the lower mantle, though at times slabs seem to flatten or terminate at about 600–700 kilometers. Deep-focus earthquakes within subducting slabs provide indirect constraints on their integrity at depth, but direct observation of slabs at great depths remains challenging.

Implications of deep subduction include its role in mantle convection patterns, the transfer of volatiles and chemical signatures into deep mantle reservoirs, and possibly influences on surface volcanic activity. It is an active area of research with ongoing debates about the ultimate fate of subducted slabs and their impact on Earth's interior.